This invention relates to a method of manufacturing diamond semiconductor and, more particularly, to a method of manufacturing diamond semiconductor with improved diamond semiconductor doping techniques.
There were attempts to artificially synthesize diamond in old days, and in 1960s it became possible to successfully syntheize diamond even under low pressures. Up to date, it has become possible to manufacture thin films of diamond under pressure close to vacuum. This has led to extensive attempts to obtain diamond semiconductor from thin film diamond and produce semiconductor devices by using such diamond semiconductor.
Compared to silicon, diamond has the following characteristics.
(1) Its carrier mobility is so higher that its operation speed is several times that in case of silicon. PA1 (2) Its band gap is as wide as 5.5 eV, and thus it is capable of operation without destruction at an overwhelmingly high temperature of 700.degree. C. compared to silicon. (With silicon, a trend of operation state deterioration arises at around 150.degree. C.) PA1 (3) It is strong against radiant rays. Particularly, it is less subject to software error due to radiation which is a problem in the super LSI. This means that it can be used even in bad environments. Thus, it is expected to be used in stringent environments such as universal space or places around a nuclear reactor. Such applications can not be expected with silicon. PA1 (1) There still is no process of inexpensively growing single crystal thin film. PA1 (2) It is impossible to obtain n-type doping. PA1 (3) There still is no estabilished etching technique for drawing complicated circuits.
For practical use of diamond semiconductor, however, there are problems to be solved such as follows.
Particularly, the problem in (2), i.e., impossibility of n-type doping, is a fatal one in the device production, which is very serious for the future practical use of diamond semiconductor.
This point will be described in further detail. Diamond itself is generally very inferior in the doping efficiency when it is used as semiconductor material. For example, the carrier concentration that is obtainable by a p-type semiconductor with 1,000 ppm of boron is only about 10.sup.16 cm.sup.-3.
Further, the constituent element of diamond is carbon, with its atomic diameter being small compared to silicon or the like. Therefore, it is impossible to use for the n-type doping such element as phosphorus which is used in the silicon process, and it is inevitable to use nitrogen with the atomic diameter close to that of carbon (see, for instance, Japanese Patent Laid-Open Publication No. Sho 4-266020). In case of using nitrogen, however, there is the following problem.
The fifth valence electron of the nitrogen atom is strongly bound to the nitrogen atom, and its releasing requires as great electron energy as 1.7 eV. Therefore, at room temperature only a very small number of electrons can be moved in the crystal, and thus effective device functions can not be obtained. For this reason, when using nitrogen atoms as dopant, it is necessary to effect the doping without causing destruction of the crystal structure and also with a high concentration. This dictates some or other contrivance for the doping process. Due to this problem, nitrogen is inconvenient as doping means. Accordingly, there has been a demand for an n-type doping technique using an element other than nitrogen.
As prior art, there has been proposed a technique of forming diamond semiconductor through vapor growth (CVD) using a heated filament process, which uses as material a liquid-phase organic compound containing lithium or a compound thereof (obtainable by dissolving lithium oxide, lithium hydroxide, lithium chloride, lithium ethylate, etc. in acetone, methanol, ethanol, altaldehyde, etc.) ( Japanese Patent Laid-Open No. Hei 3-205398). There has also been proposed a technique of forming diamond semiconductor through the heated filament CVD or various types of plasma CVD by adding lithium or a compound thereof to the material gas (such as methane) in a heating device (Japanese Patent Laid-Open Publication No. Hei 4-175295). These techniques, however, are not always clear with respect to their realization. In addition, in either technique impurities are introduced simultaneously with the diamond semiconductor thin film formation, and it is not that the diamond semiconductor having been formed is doped.